In recent years, a high density mounting technology of electronic components has been making a rapid progress in accordance with miniaturization and thinning of electronic devices. FC and BGA with the hemispherical solder bumps are used as semiconductor devices for achieving the high density mounting.
As a method for forming solder bumps on pad electrodes, generally used are a method in which pad electrodes are made to be in contact with molten solder (molten solder method), a method in which solder paste is screen-printed on pad electrodes and is re-flown (screen printing method), a method in which solder balls are mounted on pad electrodes and are re-flown (solder ball method), a method in which solder plating is applied on pad electrodes (plating method), etc. In addition, there is another known method for forming solder bumps as disclosed in Japanese Patent Unexamined Publication No. 7-114205 (FIG. 1, etc), for example.
FIG. 4 is a schematic cross section for showing the forming method disclosed in Japanese Patent Unexamined Publication No. 7-114205. It will be described in the followings by referring to the drawing.
In this forming method, first, in an inert solvent 80 which is heated to be hotter than a melting point of solder, a wafer 82 having copper electrodes 81 on a surface is immersed with the surface facing down. Then, in the inert solvent 80, solder particles 84 made of molten solder 83 are emitted upwards. Thereby, solvent bumps (not shown) are formed on the copper electrode 81 by making the solder particles 84 come in contact with the wafer 82. It will be described in more detail.
The molten solder 83 and the inert solvent 80 inside a heat tank 85 are controlled to be at temperatures slightly higher than a melting point of solder, for example, at 200° C. The molten solder 83 inside the heat tank 85 is sucked into a solder-fine-particle forming unit 87 through a solder intake pipe 86. Further, the solder-fine-particle forming unit 87 sucks the inert solvent 80 which is at the same temperature as that of the molten solder 83 through an inert solvent intake pipe 88 and mixes/agitates these two liquids for breaking the molten solder 83 so as to have it pulverized. Then, the inert solvent 80 containing the solder particles 84 is transferred to an ejection unit 90 through a mixed-solution outlet pipe 89 to be emitted upwards from nozzles 91.
The solder particles 84 in the inert solvent 80 are coated by the inert solvent 80 so that they are not to be exposed to the open air. Thus, the surfaces of the solder particles 84 maintain the metal surfaces to be in an active state. The solder particles 84 in the inert solvent 80, when in contact with the surface of the immersed wafer 82, form a solder alloy layer together with the copper electrodes 81 and stick to the surfaces thereof. Thereby, the surfaces of the copper electrodes 81 are covered by the molten solder coating (not shown). Subsequently, since the solder particles 84 are easily adsorbed to the solder coating so that the solder particles 84 in this part are stuck onto the solder coating one after another.
In the meantime, the solder particles 84 which do not stuck onto the copper electrodes 81 gradually fall down due to the specific gravity difference and deposit in the bottom of the heat tank 85. As described above, by immersing the wafer 82 with the copper electrodes 81 facing down in the inert solvent 80 where the solder particles 84 are emitted upwards, the solder bumps (not shown) can be selectively formed on the surfaces of the copper electrodes 81.
However, in the molten solder method, there is such a drawback that there is a small amount of the solder in the solder bumps and the difference in the amount is large, while it is suitable for achieving fine pitch of the pad electrodes. In the silk screen printing method, it is not suitable for achieving the fine pitch since it is likely to cause clogging and uneven solder amount when using a mask with the fine pitch, while it enables to easily form the solder bumps collectively. In the solder ball method, as the recent trend, the number of solder balls used in a single semiconductor device is extremely large and, in addition, the size of the solder balls are extremely small so that it increases the manufacturing cost. In the plating method, there is no plating liquid appropriate for lead-free solder which has recently come into use. In the forming method as disclosed in Japanese Patent Unexamined Publication No. 7-114205, there is such a drawback that the solder particles are not easily stuck to the copper electrode, i.e. the solder wetability is not great, so that it is difficult to put it into practical use.
Therefore, the object of the present invention is to provide a solder supplying method, which enables to achieve fine-pitched pad electrodes and also to obtain solder bumps with a large amount of solder and a small difference in the amount thereof, and to provide a method and an apparatus for forming the solder bumps using the same.